Rotating crops, chemistry: Resistant weeds need careful management

Rotating crops, chemistry, and control strategies can provide the basics for avoiding or managing herbicide-resistant weeds.

“Herbicide resistance is not much of a problem to Southwestern growers — unless you have it,” says Texas Extension Agronomist Todd Baughman, with the Texas Cooperative Extension atVernon.

He discussed the alarming expansion of weed resistance at the recent Concho Valley Cotton Conference at San Angelo.

Before 1982, Baughman said, only 35 weed species worldwide had been identified as resistant to herbicides. From 1990 to 1995, that number rose to 214, and currently, 313 weed species make the list.

“In Texas, we've seen resistant ryegrass, barnyardgrass, Palmer amaranth, kochia, johnsongrass, and horseweed,” he said. “Resistance is the ability of a segment of the weed population that a particular herbicide controlled at one time to survive that herbicide.”

The process is “naturally-occurring and not a mutation caused by the herbicide,” Baughman said.

Resistance may show up as requiring much higher rates for effective control. “But, some weeds have a natural tolerance and we know it's harder to control larger weeds. These situations are not the result of herbicide resistance.”

Cross resistance — weeds displaying resistance to herbicides with different modes or sites of action — also causes concern. “We might see a weed that's resistant to yellow herbicides and Staple.”

Recent reliance on glyphosate for weed control in herbicide tolerant crops has exacerbated the problem.

“Glyphosate is highly effective, and can be used multiple times on tolerant crops during the season,” Baughman said. “Often, growers apply no herbicide for the rest of the year.”

The downside is that glyphosate has no residual soil activity. “But rotation to non-Roundup Ready crops helps manage resistance and we still have effective alternative herbicide options.”

Resistance develops because of several factors, including:

No crop rotation

No herbicide rotation

Presence of a highly susceptible species

Sole dependence on herbicides for weed control

Reduced tillage

Narrow-row planting, which eliminates option of in-season cultivation

More acres per grower

Fewer herbicide modes of action

Reduced use of soil-applied herbicides

Reduced labor and equipment used in weed control programs

The way a herbicide works plays a crucial role in resistance management. “Mode of action — how the herbicide inhibits plant growth — and site of action — the exact location within the plant the herbicides binds — are important.”

Using herbicides with different modes of action or different sites of action can help manage resistance.

Baughman said producers should watch for resistance in lambsquarter, Russian thistle, horseweed, Palmer amaranth and “a long list of other weeds.”

Growers who use the same herbicide year after year select for resistance. “We may see weed shifts with reliance on one herbicide. We may see more morningglory, for instance, with reliance on Roundup.

“We've seen a slow increase in the number of weeds resistant to Roundup. It started in 1997, in Malaysia, with horseweed. Palmer amaranth resistance showed up in Georgia and North Carolina and is suspected in several other states.”

He said horseweed features small, windblown seed that move easily from one field to another. Palmer amaranth has male and female plants and may transfer genes through pollination, which “creates the potential to develop resistance quickly.”

But, the situation is far from hopeless.

Baughman suggests several best management practices to deal with herbicide resistance. Recommendations include:

Use different herbicides with different modes or sites of action

Rotate crops

Use combinations of pre-plant incorporated, pre-emergence, and post-emergence applied herbicides

Target weed species and select the proper herbicide or control strategy

Baughman said farmers who notice a pattern of weed escapes in a field may be witnessing an application error, rather than resistance.

Farmers can expect few new herbicides with different modes of action.

“We're losing some to the high cost of re-regulation,” he said. “I don't expect any silver bullets. Palmer amaranth is the most common weed in Texas and resistance has already been documented in the Southeast. So, we know that glyphosate resistance can occur.”